In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential to prepare an image panel on the surface for generation of a latent image. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. The light radiation exposes the charged photoconductive member for selective dissipation of the uniform charge. The selective dissipation of the uniform potential forms an electrostatic latent image corresponding to the informational areas contained within the original document. The raster output scanner (ROS) is a component of the exposure station that directs light onto the photoreceptor to form the latent image in the xerographic process. After the electrostatic latent image is generated on the photoconductive member, the photoconductive member is rotated to a developer station containing developer material comprised of toner particles that adhere triboelectrically to carrier granules. A bias voltage transfers the toner particles from the carrier granules to the latent image to form a toner powder image on the photoconductive member. The photoconductive member is rotated to a transfer station where the toner powder image is transferred from the photoconductive member to a copy medium. The copy medium is transported to a fuser station where the toner particles are heated and pressed into the copy medium to permanently affix the powder image to the copy medium.
The foregoing generally describes a typical black and white electrophotographic printing machine. With the advent of multicolor electrophotography, a plurality of image forming stations is used to overlay color separated images in pixilated patterns for generation of three or four color images. One example of the plural image forming station architecture utilizes an image-on-image (IOI) system in which the photoreceptive member is recharged, reimaged and developed for each color separation. The charging, imaging, developing and recharging, reimaging and developing of the latent image to impose different color toner particles on the latent image may be performed in a single cycle or in multiple cycles. The multiple pass-machines produce only one color toner image during each pass of the photoreceptor and the image is transferred to the copy medium on the last pass through the machine. The single pass architecture offers a potential for high throughput, but the machine is expensive as it requires a charging, exposure and development stations for each color. While the multi-pass architecture is simpler and less expensive, its throughput is less than the single pass architecture.
Regardless of the architecture, the development of color separation images following the first application of toner particles is complicated by the attenuation of the exposing light by the toner particles. That is, the photoreceptor needs to receive the light in order to alter the voltage at the exposed photoreceptor area. If the light does not penetrate the toner particles, the photoreceptor is not set to a voltage level appropriate for attracting developer material in the amount required for good color quality. For example, if too little cyan toner color is applied over the yellow and magenta toner colors previously applied to a latent image, the resulting color does not properly reproduce the color in the original image.
For good quality imaging, a color image-on-image developer station must deliver consistent toner densities through the entire tone reproduction curve (TRC). Benchmark color systems control the TRC using photoreceptor charge voltages, developer bias voltages, and ROS intensity as actuators. Because different points along the TRC are sensitive to different actuators, the reproduction process can be controlled along the entire curve to produce consistent toner densities. However, systems that control the TRC by altering the power levels of the ROS need careful control, otherwise, the variations in the reproduced colors may be unacceptable. One may monitor the density of applied toner particles following application of the toner particles at a development station with densitometers that are used to regulate the bias voltages for development stations. This bias is the same across the entire photoreceptor; however, the voltage difference driving toner to the photoreceptor may vary substantially between bare and toned areas of the photoreceptor, if light attenuation by the toner layer is too large. Control of the TRC based on measurements of development on bare photoreceptor areas case does not adequately reproduce colors on photoreceptor areas having multiple layers of toner obtained from multiple development stations. To address this issue, control of ROS intensity may be used, but this method requires careful balancing. A wider range of ROS intensity may give better control of the TRC on bare photoreceptor areas; but, a narrower range may be required for overall color stability. Because the voltage difference due to ROS attenuation is highly variable, depending on charge level and photoreceptor properties, a constant range of ROS power either unnecessarily constrains toner density control, or allows too much variation in image-on-image density. Thus, variation of the ROS power range is required. The range must be adapted to the specific machine configuration, which includes photoreceptor, ROS, and exposure geometry. Too much, or too little, ROS power variation makes overall color stability unacceptable.